Switch the default version when undefined from ~dev to dev

[kit-ty-kate]

Makes it easier to mix pin packages and packages from the repository without having to set the version again when encountering an issue.

When you take the development version of something you expect more often to be ahead of the latest release rather than the opposite.

Issue encountered by @bikallem

[bikallem]

Not sure if the issue belongs to this PR, but here is another issue:
I did opam pin . --with-version 0.6.1 on root dir of repo/branch https://github.com/bikallem/resto/tree/no-lwt-4.12.0%2Bdomains. However, it seems opam pin doesn't respect the dependencies of opam packages. That is resto should be pinned/installed first and then resto-lwt and so on and so forth. However, this doesn't seem to be happening, I get the following error on my first try with the `opam pin . --with-version 0.6.1' command,

#=== ERROR while compiling resto-lwt.0.6.1 ====================================#
# context     2.1.1 | linux/x86_64 | ocaml-variants.4.12.0+domains | pinned(git+file:///home/bikal/ocamllabs/resto#no-lwt-4.12.0+domains#2b60417c)
# path        ~/.opam/4.12.0+domains/.opam-switch/build/resto-lwt.0.6.1
# command     ~/.opam/opam-init/hooks/sandbox.sh build dune build -p resto-lwt -j 47
# exit-code   1
# env-file    ~/.opam/log/resto-lwt-133343-63c2fb.env
# output-file ~/.opam/log/resto-lwt-133343-63c2fb.out
### output ###
# File "src/dune", line 11, characters 12-17:
# 11 |  (libraries resto lwt lwt.unix cohttp-lwt cohttp-lwt-unix conduit-lwt-unix))
#                  ^^^^^
# Error: Library "resto" not found.
# Hint: try:
#   dune external-lib-deps --missing --no-config --root . --ignore-promoted-rules --default-target @install --always-show-command-line --promote-install-files --release --only-packages resto-lwt -p resto-lwt --profile release -j 47 @install

I need to execute opam pin . --with-version 0.6.1 several times for the packages to be pinned correctly. Each time the error is because dependent packages are not pinned first. Could this be addressed too in this PR or is this a separate issue altogether?

Note: I haven't tested this PR yet. The error description above is with stock opam 2.1.1

[kit-ty-kate]

That is resto should be pinned/installed first and then resto-lwt and so on and so forth.

@bikallem I’m not seeing the dependency to resto in your resto-lwt package anywhere